1. Field of the Invention
The present invention relates to capacitive sensors, and more specifically relates to a sensor structure in which tilt, acceleration, etc., can be detected on the basis of a capacitance change between a fixed electrode and a movable electrode.
2. Description of the Related Art
FIG. 14 shows a known capacitive sensor which uses a capacitance change.
The sensor shown in FIG. 14 includes a pair of plates 1000 and 1003 which face each other. Electrodes 1001 and 1002 are formed on the plate 1000, which is fixed, and an electrode 1004 is formed on the plate 1003, which is movable and has flexibility. The electrode 1004 serves as a movable electrode which rotates with respect to the fixed electrodes 1001 and 1002. In addition, a weight 1005 is attached to the flexible plate 1003, and when the sensor tilts or when an external force is applied to the sensor, the flexible plate 1003 is distorted due to the weight 1005, so that the capacitances P between the fixed electrode 1001 and the movable electrode 1004 and between the fixed electrode 1002 and the movable electrode 1004. Accordingly, the tilt, the external force, etc., is detected by determining the capacitance changes.
In the above described construction, the movable plate 1003 is initially deflected due to the weight of the weight 1005. Thus, the gap between the movable plate 1003 and the fixed plate 1000 cannot be easily maintained.
In addition, signals based on the detected capacitances include not only the capacitance changes due to the distortion of the movable plate 1003 but also offset capacitances corresponding to the capacitance increase caused by the displacement of the movable electrode 1004. The offset capacitances are large when the tilt angle 0 of the sensor is small since the moving direction of the movable plate 1003 is close to the direction of gravity, and are small when the sensor is in an approximately vertical position since the moving direction of the movable plate 1003 is close to the direction perpendicular to the direction of gravity.
Accordingly, when the sensor is tilted, a capacitance Pa of a capacitor A constructed by the movable electrode 1004 and the fixed electrode 1001 and a capacitance Pb of a capacitor B constructed by the movable electrode 1004 and the fixed electrode 1002 vary in accordance with the tilt angle xcex8 of the sensor, as shown in FIG. 15 along curves having maximum values when the tilt angle xcex8 is about xc2x130xc2x0. Accordingly, when the tilt of the sensor is detected, the detection range is xc2x130xc2x0, which is extremely small. In addition, as shown in FIG. 15, the difference between Pa and Pb (Paxe2x88x92Pb) varies along an xe2x80x98Sxe2x80x99 shape having maximum and minimum values when the tilt angle xcex8 of the senor is xc2x1xcex80 (that is, xc2x1F0 in terms of the force applied to the weight 1005). The tilt angle xcex80, at which Paxe2x88x92Pb has the maximum and minimum values is generally about 60xc2x0 (although this varies with the construction of the sensor), and even when the tilt of the sensor is detected on the basis of the difference between the capacitances, the detectable angle range is about xc2x160xc2x0, which is still small.
In addition, when the sensor is attached to an external device, there is a problem in that the sensor cannot be attached with sufficient freedom since the detection range of the tilt angle, etc., is small.
Furthermore, since the capacitances P vary in accordance with a force applied to the weight 1005 along curves having maximum values, when the tilt angle xcex8 is determined on the basis of a capacitance change, calculations to compensate for the capacitance change due to the vertical displacement of the movable electrode 1004 must be performed. Thus, complex calculations must be performed by using an expensive processing circuit, and the detection accuracy is degraded due to the compensation calculation.
In view of the above-described situation, an object of the present invention is to provide a capacitive sensor which can be attached with sufficient freedom and which can detect a force applied to a weight on the basis of a capacitance between a fixed electrode and a movable electrode-without compensating for the vertical displacement of a movable member.
In order to attain this object, a capacitive sensor according to the present invention includes a fixed board which is provided with a fixed electrode unit consisting of one or more electrodes; a movable member provided with a movable electrode unit consisting of one or more electrodes which faces the fixed electrode unit with a gap therebetween, the movable member being rotatable with respect to the fixed board; a weight provided on the movable member; and a retaining element which is disposed between the fixed board and the weight and retains the movable member in a rotatable manner. At least one of the movable electrode unit and the fixed electrode unit includes a plurality of electrodes, and the movement of the movable member is detected on the basis of capacitances between the one or more electrodes of the movable electrode unit and the one or more electrodes of the fixed electrode unit change in accordance with the movement of the movable member.
In the above-described construction, the movable member smoothly rotates (or seesaws) around a retaining point at which the retaining element retains the weight. Since the movable member is directly or indirectly retained by the retaining element, the movable member is not displaced due to the weight of the weight and a gap between the movable member and the fixed board is maintained constant at the retaining point irrespective of the tilt of the sensor. Therefore, the capacitance between the fixed electrode unit and the movable electrode unit changes linearly in one-to-one correspondence with the tilt angle of the sensor over the range from xe2x88x9290xc2x0 to +90xc2x0 at a maximum. In addition, even when the sensor is attached in an inclined manner, the tilt from the position at which the sensor is attached, an external force, etc., can be detected. Therefore, the sensor can be attached with sufficient freedom. In addition, since the capacitance change linearly in one-to-one correspondence with the tilt or the external force and do not have maximum values, the detection range can be increased. Furthermore, since the movable member is retained by the retaining element, the movable member is not displaced in the vertical direction. Accordingly, calculations to compensate for such a displacement can be omitted, so that the detection accuracy can be prevented from being degraded due to the compensation calculations.
The retaining element can be formed integrally with the fixed board. In such a case, the positional relationship between the retaining element and the fixed electrode unit can be set more accurately and the detection accuracy of the capacitance can be improved compared to the case in which the retaining element is formed separately. In addition, the number of components can be reduced, so that the costs can be reduced.
In this case, the movable member preferably have a concave portion for receiving an end portion of the retaining element at a position where the movable member is in contact with the retaining element. Accordingly, the retaining point at which the retaining element retains the movable member is fixed by the concave portion, so that the positional accuracy of the movable member and the fixed board can be improved. In addition, when the movable member and the fixed board are positioned such that they are parallel to each other while the sensor is in a horizontal position, that is, in a neutral state, offsets are not easily generated. Furthermore, since the movable member always rotates around the same retaining point, stable detection accuracy can be obtained.
The retaining element may also be formed integrally with the weight. In such a case, the positional relationship between the retaining point of the retaining element and the gravity center of the weight can be set more accurately. For example, when the retaining point is set such that it coincides with the gravity center of the weight in a plan view, the movable member can rotate with respect to the fixed electrode unit in a balanced manner.
In this case, the fixed board preferably have a concave portion for receiving an end portion of the retaining element at a position where the fixed board is in contact with the retaining element. Accordingly, the positional accuracy of the fixed board and the movable member can be improved and offsets are not easily generated in the neutral state. In addition, since the movable member always rotates around the same retaining point, stable detection accuracy can be obtained.
The retaining element may also be formed integrally with the movable member. In such a case, the fixed board preferably have a concave portion for receiving an end portion of the retaining element at a position where the fixed board is in contact with the retaining element. Accordingly, the positional accuracy of the movable member and the fixed board can be improved and offsets are not easily generated in the neutral state. In addition, since the movable member always rotates around the same retaining point, stable detection accuracy can be obtained.
In addition, the movable member may be biased in a direction away from the fixed board by the retaining element. In such a case, even when the sensor is turned over and the weight is positioned at the bottom side of the fixed board, the movable member does not come away from the retaining element. Accordingly, the tilt of the sensor, an external force, etc., can be detected irrespective of the orientation in which the sensor is attached.
The capacitive sensor may further include a retaining member which retains the movable member and a plurality of flexible connecting members which connect the movable member to the retaining member. In such a case, the movable member can easily rotate, so that the amount of capacitance change can be increased and the detection accuracy can be improved. Since the movable member is retained by the retaining element, the connecting members do not substantially receive the weight of the weight. Thus, the widths of the connecting members can be reduced so that the movable member can rotate more easily. Accordingly, the amount of capacitance change can be increased and the sensitivity of the sensor can be increased accordingly.
The retaining member may be placed on the fixed board. In such a case, the gap between the movable member and the fixed board is determined by the size of the retaining element. Since a spacer, etc., for providing a gap between the retaining member and the fixed board is not necessary, the number of components can be reduced. In addition, when the retaining member and the movable member are arranged in the same plane and are connected to each other with a plurality connecting members, the movable member is biased in a direction away from the fixed board by the retaining element. Thus, even when the sensor is turned over, the movable member does not come away from the retaining element. Accordingly, the tilt of the sensor, an external force, etc., can be detected irrespective of the orientation of the sensor.
The retaining member, the connecting members, and the movable member may be composed of a conductive material. In addition, a connecting electrode may be provided on the fixed board and the retaining member may be placed on the connecting electrode with or without a metal spacer therebetween. In such a case, the ground potential and electrical signals such as a drive signal can be supplied directly, or via the metal space, from the connection electrode to the movable member. Accordingly, it is not necessary to provide an additional electric line for signal supply which connects the connection electrode and the movable member, and the parasitic capacitance of such an electric line is not generated.
The capacitive sensor may further include a cover which covers the peripheral region of the fixed board and which pushes the retaining member toward the fixed board. In such a case, the sensor can be protected from dust, moisture, careless handling, etc. In addition, since the retaining member is clamped between the cover and the fixed board when the cover is fixed to the sensor, it is not necessary to apply an adhesive for fixing the retaining member. Accordingly, the assembly accuracy can be improved and productivity can be increased.
In addition, a packing may be disposed between the cover and the fixed board. In such a case, foreign matter, flux, moisture, etc., can be prevented from flowing to the inner region of the cover.